A polymer electrolyte fuel cell basically includes a polymer electrolyte membrane having proton conductivity and a pair of electrodes arranged on both surfaces thereof. Each of the electrodes includes a catalyst layer mainly made of platinum or platinum-group metal catalyst, and a gas diffusion electrode which is formed on an outer surface of the catalyst layer and which serves to supply a gas and to collect a current. A body formed by integrating the electrodes and the polymer electrolyte membrane is called a membrane electrode assembly (MEA). A fuel (hydrogen) is supplied to one of the electrodes and an oxidizer (oxygen) is supplied to another of the electrodes, thereby performing power generation.
In the fuel electrode to which hydrogen is supplied as the fuel, a reaction expressed by the following (formula 1) occurs, and protons and electrons are produced from hydrogen. Further, in the oxidizer electrode to which oxygen is supplied as the oxidizer, a reaction expressed by the following (formula 2) occurs, and water is produced from oxygen, protons, and electrons. In this case, the protons move from the fuel electrode to the oxidizer electrode by passing through the polymer electrolyte membrane. Further, the electrons move from the fuel electrode to the oxidizer electrode by passing through an exterior load. In this process, electric power is obtained.fuel electrode: H2→2H++2e−  (formula 1)oxidizer electrode: ½O2+2H++2e−→H2O  (formula 2)
A theoretical voltage of the fuel cell is about 1.23 V, but the fuel cell is often used at the voltage of 0.7 V in a normal operation state. The voltage drop occurs in relation to various losses (polarization) in the fuel cell.
For the polymer electrolyte membrane, a perfluorosulfonic acid polymer electrolyte membrane typified by Nafion (trademark of DuPont) is widely used. In the perfluorosulfonic acid polymer electrolyte membrane, for conducting the protons, water moving while entrained with the protons is necessary. Accordingly, in order to obtain sufficient proton conductivity, a moisture content of the polymer electrolyte membrane has to be increased. In this connection, it is known that, when the fuel cell is operated when the moisture content is low, a voltage of the fuel cell is reduced.
Further, it is known that, in a case where a potential of the oxidizer electrode of the fuel cell with respect to a potential of the fuel electrode is set to a high value equal to or higher than 0.8 V, an oxide layer is formed on a platinum catalyst surface used for the oxidizer electrode, thereby causing degradation in catalytic activity.
In order to suppress the voltage reduction of the fuel cell due to the above-mentioned causes and to keep characteristics of the fuel cell in a high and stable state from immediately after start of the operation, an activation method for the fuel cell is conventionally proposed.
The activation of the fuel cell herein refers to an operation of increasing a moisture content of the polymer electrolyte membrane and reducing loss (polarization) by removing an oxide layer formed on the catalyst surface.
Japanese Patent Application Laid-Open No. 2005-093143 discloses a method of activating a fuel cell by short-circuiting opposite electrodes of the fuel cell.
The voltage of the fuel cell is retained around 0 V, thereby reducing and removing the oxide layer formed on the platinum catalyst surface. Further, by allowing passage of a current generated by the fuel cell reaction, production of water is promoted, thereby humidifying the polymer electrolyte membrane.
In the activation method, it is impossible to pass through the fuel cell a current equal to or larger than a maximum current (hereinafter, referred to as limiting current) passed at a time of the steady operation.
Japanese Patent Application Laid-Open No. 2006-040598 discloses a method in which, in a fuel cell for supplying an organic fuel such as methanol to a fuel electrode, while an inert gas is supplied to an oxidizer electrode, the fuel cell is connected to an exterior power source such that potentials of the fuel electrode and the oxidizer electrode are inverted, thereby humidifying a polymer electrolyte membrane.
In this case, in the oxidizer electrode, by the protons moved from the fuel electrode through the polymer electrolyte membrane and the electrons supplied from the exterior power source, a hydrogen production reaction as expressed by a formula 3 occurs.oxidizer electrode: 2H++2e−→H2  (formula 3)
In a process of the above-mentioned reaction, along with the movement of the protons, water is spread in the polymer electrolyte membrane, thereby performing the humidification. Further, since the potentials of the fuel electrode and the oxidizer electrode are inverted, the potential of the oxidizer electrode is retained to be lower than that of the fuel electrode, thereby promoting reduction and removal of the oxide layer formed on the platinum catalyst surface.
However, the activation described in Japanese Patent Application Laid-Open No. 2006-040598 is performed for a purpose of humidifying the polymer electrolyte membrane, and is still insufficient in terms of a point of enhancing the characteristics of the fuel cell in the entire current region used at a time of the steady operation.